Unfortunately your little EXE only loads files [and folders] to Mp3tag- thus replacing the old file list. But if it is clicked with the CTRL being pushed in, then the files / folders are added. And so the full trick is to: set a hot key like CTRL + 1 for it, because then the CTRL is being pushed as you execute it

[But I am pretty much sure I am just dreaming and tomorrow this will somehow turn out to work only sometimes and my problem still will not be solved]

[...]
[But I am pretty much sure I am just dreaming and tomorrow this will somehow turn out to work only sometimes and my problem still will not be solved]

And here I am- back to square one

It seems that this solution works- but can stop at any given attempt of loading files

Sometimes I can add files 10 times and sometimes 5 or just 1. But inevitably Mp3tag gets somehow corrupted and the Button instead of adding, just replaces the current list with whatever is selected. Then I have to close Mp3tag and just reopen it- then the Button and CTRL duet again work properly. [And so this fixing by the means of reopening of Mp3ag tells us that it is the fault of Mp3tag and not the FreeCommander or dano's Button]

And I did make sure it was not a case of a broken CTRL on a keyboard- I repeatedly experienced this break down also when using two different virtual keyboard [thus pressing CTRL + 1 evoking my button with my mouse]

If I go that way with the parameter %ActivSelAsFileUtf8% "path to exe", then the Path in Tag-Panel in MP3tag is set to the folder of FreeCommander. So if I want to make a playlist in MP3TAG, the default Save-Path is not the path where the Files are.
But if I drag and drop the files from FreeCommander to MP3TAG and make then a playlist, then the default Save-Path ist set to the real folder where the files are.

Only thing I could change is that my program simulates a Ctrl key press.
Best would be if Florain adds a switch to the Mp3tag command line.

How about creating second versions of your little piece of software- an alternative one, that would add and not replace? The Mp3tag users could choose then whichever they like. Or just use both of them, depending on a situation